During a printing operation, for the purpose of process monitoring, it is usual to provide printed control strips with colored test patterns outside the subject on sheets or webs to be printed. These control strips, whose longitudinal direction is transverse with respect to the transport direction of the printing material, contain a set of measurement areas which are repeated periodically in the longitudinal direction. A specific characteristic variable that characterizes the printing quality can be measured on each measurement area.
For example, in the case of four-color printing, a typical configuration of a control strip is a sequence of measurement areas which are printed with the full-tone colors black, cyan, magenta and yellow, in order to permit a measurement of the respective ink density. By using such ink density measurements, conclusions about the ink supply in the inking unit of the press can be obtained, and its setting can be optimized when starting up the press. In continuous operation, the measurements are used for the purpose of monitoring and documenting the maintenance of required set points. In this case, the periodic repetition of the measurement areas in the control strip results from the need to check the homogeneity of the ink supply over the width of printing material. The periodic spacing of the measurement areas depends on the zone subdivision of the ink supply over the printing material width in the inking unit of the press. These substantially periodic areas can be assigned further areas for the measurement of further variables characteristic of the printing process, and it is possible for further areas to be distributed both with the same or different periods and also irregularly.
For measurements of this type, it is known to arrange a measuring apparatus in a press. The main components of the measuring apparatus include a light source for illuminating the printed product, a camera aimed at the printed product for recording an image from an extract of the printed product, and an electronic evaluation unit. In this case, the aforementioned extract of the printed product has a control strip of the type mentioned above. The camera has an electronic two-dimensional image sensor, from which the evaluation unit reads the images acquired and from these determines characteristic variables of the printed product, for example, density values of the printed inks. Apparatuses of this type and methods for their operation are described in EP 0 741 032 B1 and DE 195 38 811 C2.
In order to achieve high accuracy in the determination of the characteristic variables of interest, a plurality of error sources have to be taken into account when performing measurements of the type in question, and appropriate corrections have to be provided. These error sources include in particular the stray light falling onto the image sensor during the measurement. Other error sources include inaccuracy in the references for the zero light intensity and the intensity of the light shone onto the printed product by the light source.
In order to take into account the dark current of photoelectric sensors, which is the output signal of the sensors when there is no illumination, an image which corresponds to a completely black area can be recorded as a black reference, and can be included in a suitable way in the image evaluation. Furthermore, the intensity of the light reflected from a white (i.e., unprinted) area is of interest for the image evaluation, since this is involved in particular in the very important evaluation of the characteristic variable ink density.
Thus, in EP 0 741 032 B1, a measuring apparatus arranged in a press is described in which a black reference area and a white reference area each having an exactly known spectral reflectance characteristic are arranged laterally outside the movement path of the printing material. For the purpose of calibrating the image sensor, during the commissioning of the apparatus, a plurality of images of each of the two reference areas is recorded and these, the zero point and the maximum value of the intensity, are set iteratively in the evaluation unit based on the recorded images. As an alternative to this, in DE 195 38 811 C2, the black reference proposed is an image recorded with the camera objective covered or an image recorded from a black area without illumination, and the white reference proposed is an image recorded from an unprinted region of the printing material with illumination.
An error source with a considerable influence on the measurement accuracy is the scattered light, i.e., the incidence of light on a sensor element of the image sensor which does not come from that section of the area of a measuring area on a printed product associated with the relevant sensor element. Therefore, attempts are made to determine a measure of the scattered light incident on the image sensor in order to be able to carry out a suitable correction during the processing of the intensity signals supplied by the image sensor.
For this purpose, it is proposed in EP 0 741 032 B1 to provide specific test patterns for the acquisition of scattered light data, to record a plurality of images of these test patterns in advance and to use these to calculate a compensation curve, from which correction factors for the elimination of the scattered light influence can be taken during the processing of the images of colored measuring areas.
DE 195 38 811 C2 instead teaches the application of digital filtering of the recorded images to eliminate the scattered light influence. The filter coefficients are calculated in accordance with a complicated mathematical method by using the intensity transition at a black/white edge contained in the control strip with colored measuring areas. In this case, a mathematical model for the distortion of an ideal black/white edge by the light scatter, known as an unsharp function, is used as a basis for the calculation.
The known concepts for reducing measurement errors during the characterisation of printed products appear overall to be complex and time-consuming. Application in continuous printing operation would therefore permit only a relatively low measuring frequency, which at least calls into question the use of the measured data for the regulation of processes in the press. On the other hand, given only one acquisition of reference data and setting of compensating and correction parameters before the start of printing operation, the accuracy of the measured data during relatively long operation is not ensured.